The main argument is one of degree. Hickel isn't contradicting whether the trend is down or up, and indeed it's somewhat irrelevant whether the trend is down. What he's pointing out is that the infographic is quite misleading in the impression that it gives to the lay person. Extreme poverty has a specific definition that isn't very useful when trying to understand the big picture, because "extreme poverty" is below the line where you get enough food to live, and it's very far below the line where you can expect a reasonable life. The high level message that the infographic conveys is that poverty in general is nearly gone, where the reality of global poverty is quite different, and not going away as fast as "extreme poverty". Saying that Gates & Pinker never claimed poverty is solved is hiding behind a technicality. The social media they're sharing has a very clear implication, and most people aren't digging into the details and minutiae of this argument.
It would settle this whole thing to put all the trend lines for extreme poverty vs regular poverty vs lower class vs middle class vs upper class all on one graph. No matter what your opinion is here, it's true that using only extreme poverty on a single graph is cherry-picking and likely to not convey an accurate impression of the overall situation.